telephone and user) Can be added to any SA253x- application Enables use of low cost 8 - Loudspeaker High output power with low distortion Entirely powered from the telephone line No influence of loudspeaker amplifier in AC-impedance, sidetone cancellation or DC-mask Loudspeaker volume adjustable by potentiometer Excellent sidetone cancellation of SAS253x circuits enables high loudspeaker volume without howling between handset microphone and loudspeaker

The amplifier is a simple, low cost add-on circuit to the SA253x single chip telephone family. It is supplied by the line current and amplifies the receive signal, taken from either or RO2 (Receive outputs), see Fig.1. Compared to more complex "real" handsfree circuits there is no channel switching or anti-larsen circuit implemented, therefore the receive and transmit gains must be lower to avoid howling. However, at short distances between user and telephone (<2m, which is adequate for normal office work) there is even full duplex "handsfree" operation possible, when the handset is placed next to the telephone with the microphone facing to the user. Due to the excellent sidetone cancellation of the SA253x circuits, howling between handset microphone and loudspeaker will only occur, when both are put very close to each other. In normal operation, no howling will occur, even with maximum loudspeaker volume. Due to the unique structure of the SA253x family's line adaption, adding the amplifier has no influence on the important PTT related parameters, like AC-impedance, DC-mask, etc. (see Pt. 5.1 below).

Note: all subsequent component numbering refers to the schematic, shown in Pt. 9

5.1 Amplifier supply: Comparison of SAN2202a and SAN2202b: The following table describes the difference between the two types of amplifier supply: while the simple SAN2202a supply shows slight influence in maximum sending level and "Make"-resistance in pulse dialing , the SAN2202b (3 extra components) supply has no influence in these parameters and has a more stable maximum output driving capability at high line currents (see & 5).